1. Field of the Invention
This invention relates to the detection of radio frequency (RF) data packets and symbol timing recovery in such packets, and to the actuation of both digital and analog GO functions in response to detected data packets.
2. Description of the Related Art
The present invention has particular application to the Bluetooth™ wireless technology data communication system, although it is not limited to this technology. The Bluetooth specification, established by the Bluetooth SIG, Inc., integrates well-tested technology with the power-efficiency and low-cost of a compliant radio system to stable links between mobile computers, mobile phones, portable hand-held devices and the like, and connectivity to the Internet.
The Bluetooth receiver includes a received signal strength indicator (RSSI) that senses the signal strength of a received data packet and, in response, governs an automatic gain control function to ensure a constant signal level at the input of an analog-to-digital converter (ADC) that converts the signal to a digital format, in preparation for down conversion to base band. To avoid overdriving the circuit with too high a gain, it is desirable that the RSSI operate only when a data packet is actually present. Accordingly, it is important that an accurate detection of a received data signal be made. It is also important that this measurement be made very rapidly, since an incoming data packet has only a 4 or 5 bit preamble. The Bluetooth specification calls for a 4 bit preamble of alternating +1's and −1's based upon the polarity of the first bit in the immediately following synchronization word. If the first bit of the synchronization word is a +1, the last bit of the preamble will be a −1, and vice versa. Thus, a total of 5 known bits are present at the beginning of each burst. It would be useful to know when a desired signal is present at the antenna input, so that its power could accurately be determined and the receiver gain set prior to reception of the desired signal's data. Such knowledge of the start of a burst could also be used to simplify other receiver functions.
Complicating the accurate detection of when a desired data packet has been received is the presence of various interferors, including adjacent channel interference (ACI), co-channel interference (CCI) and noise. If the data packet cannot be distinguished from these sources of interference, signal reception will be lost.
Bluetooth systems use an all-digital baseband receiver to recover transmitted symbols. A received analog Bluetooth signal is sampled by the ADC, usually at a sampling rate that is a multiple of the symbol rate. After further processing of the baseband signal, the symbols have to be estimated by using the correct phase of the sampling clock. Bluetooth type systems are designed to be very low cost, requiring signal processing algorithms that are computationally limited, yet efficient.
A correlation scheme that has been used for STR in a receiver that conforms to the Advanced Television Systems Committee, Inc. Vestigal Side Band standard, intended for terrestrial broadcast of high definition television (HDTV), uses a 4-symbol preamble (+1 −1 +1 −1) at the beginning of every line of a frame. Received symbols are cross-correlated with the 4-symbol preamble to estimate the best phase of the receiver clock as the phase that produces a maximum cross-correlation. Since the correlation has a maximum (+4) in only one case, the receiver attempts to adjust the phase of the sampling clock to attain this maximum value. In practice, clock phase adjustment is performed using a phase locked loop (PLL). Bluetooth, however, is a burst type transmission system with only one preamble transmitted per burst, which can be either (+1 −1 +1 −1) or (−1 +1 −1 +1). The use of a PLL to adjust the phase of the clock would add to receiver complexity and increase costs. Furthermore, a PLL implies a continuing input, whereas a Bluetooth transmission employs only one preamble per burst.